Typical methods of induction motor control include V/F control for controlling the voltage-frequency ratio of the induction motor, and slip-frequency control for controlling the current and slip frequency of the induction motor. In both of these methods the control loop is composed of a circuit, such as a analog or digital circuit, which is capable of performing only a single function. It is therefore difficult to modify the system design and to execute a complicated control operation, and the system cost is high.
In recent years, inexpensive, multiple-function high-speed microcomputers have become readily available for use in controlling induction motors in digital fashion. Such digital control is advantageous in that it reduces system cost and makes it easier to execute complicated control operations and to modify the design. Nevertheless, the conventional digital control systems call for an expensive AD converter, a microcomputer having a large number of bits, and numerous, comparatively time-consuming multiplication process steps, in order to control the induction motor in a highly precise manner. The reason is as follows.
In slip-frequency control, the steps include deriving a difference, known as the slip speed, between a commanded speed and the actual motor speed, applying an error voltage, indicative of this difference, to a proportional integrating circuit for processing, and then controlling the induction motor in a predetermined manner using the output of the proportional integrating circuit. With the conventional systems, such slip-frequency control is performed digitally by subjecting an analog speed command voltage and an analog motor speed signal to an AD conversion to obtain the corresponding digital values, finding the difference between these two digital values by means of a digital operation, and then subjecting the resulting difference value to a proportional integration operation. If we assume that eight bits are necessary to express the difference value accurately, then at least 16 bits would be required to express the speed command and the actual motor speed since both of these quantities are much larger than the difference value. In other words, to express the difference value (slip speed) accurately, the conventional method calls for a large number of bits to express the commanded speed and motor speed, as well as for a highly accurate AD converter for converting the commanded speed and actual motor speed voltages into digital signals. Furthermore, the proportional integration operation which must be performed can be expressed as follows: EQU T=K.sub.1 (.sub.C -V)+K.sub.2 .SIGMA.(V.sub.c -V) (1) EQU .SIGMA.(V.sub.c -V)=.SIGMA.(V.sub.c -V)+(V.sub.c -V) (2)
where the commanded speed is V.sub.c, the actual motor speed is V, and the proportion constants are K.sub.1, K.sub.2. The foregoing operation cannot be executed at high speed with the processing capability of a microcomputer.